J. D. Dana on the Classification of Crustacea. 17 
. 2. Á diminution of force as compared with the size of the 
Strueture, leading to an abbreviation or obsolescence of some 
cireumferential organs, as the posterior thoracic legs or anterior 
antenns, or the abdominal appendages (where such appendages 
exist in the secondary type embracing the species). "These cir- 
cumstances, moreover, are independent of a degradation of in- 
telhgenee, by an extension of the sphere of growth beyond the 
proper limits of the sphere of activity. | 
VIL. A classification by grades, analogous to that deduced for 
Crustacea, may no doubt be made out for other classes of animals. 
But the particular facts in the class under consideration, are not 
to be foreed upon other classes. "Thus, while inferiority among 
Crustacea is connected with a diminished number of annuli ce- 
phalieally absorbed (for the senses and mouth), it by no means 
follows, that the Insecta, which agree in the number of cephalic 
annuli with the lower Crustacea, are alhed to them in rank, or 
inferior to the higher species. On the contrary, as the Insecta 
pertain to à distinct division, being aerial instead of aqueous 
animals, they can be studied and judged of, only on principles 
deduced from comparison among insects themselves. "They are 
not subject to Crustacean laws, although they must exemplify 
beyond doubt, the fundamental idea at the basis of those laws. 
The views which have been explained, lead us to a modifica- 
tion, in some points, of the classification of Crustacea. "The 
question, whether the eyes are pedicellate or not, upon which the 
names Podophthalmia and Edriophthalmia are based, proves to 
be one of Secondary importance. And although still available 
in distinguishing almost infallibly the species of the first type, 1t 
is far from rendering it necessary or natural to embrace to- 
gether under à common division the species that have sessile 
eyes (so-called Edriophthalmia), as done by most writers on this 
subject. 
''he term Decapoda, in view of these principles, has a higher 
signification than has been suspected since by expressing the 
number of feet, it implies the number of cephalie annuli charac- 
terizing the species. It would not be employing it inconven- 
iently, therefore, if it were extended to embrace all the Podoph- 
thalmia, or all species of the first type, including the Mysis and 
Squilla groups. 
For a like reason, the term T*íradecopoda has a high signifi- 
cance, as applied to the species of the second type. The Lieu 
of the 'Trilobita still remains in doubt. 'The Cirripedia and 
Entomostraca, third and fourth types, stand properly on nearly 
the same level. 
